Introduction: The research has been done to determine the effect of using Internet on mental health and life quality of both male and female high school students of Tehran, Iran.Method: This was a correlation and causal-comparative research which its statistical population included all male and female middle school students of zone 8 and zone 15 of Tehran, Iran. 736 individual were selected from the statistical population by multistage cluster sampling. Research tools were Young Internet Addiction Test and short form of Anxiety, Depression and Stress Scale (DASS-21) for children.Results: Results showed that as use of Internet increases, the degree of anxiety, depression and stress of the students will increase; thus, their life quality will decrease. First grade female students use Internet more than their male counterpart, while pre-university male students use Internet more than their female counterpart.Discussion and Conclusion: It can be concluded from the results that we should pay more attention to students' use of Internet because uninhibited use of Internet might increase some psychological disorders.